1. Field of the Invention
The present invention relates to an optical communication system for transmitting and receiving a signal by using a plurality of lights having different wavelengths, and an optical amplifier used for such a system.
2. Related Background Art
An optical fiber transmission system which uses an optical fiber as a transmission line has recently been used in a field of a main channel, a local area network (LAN) or the like to take the place of a prior art transmission system which uses a coaxial cable, because of many advantages such as wide band, low loss and resistance to electromagnetic induction. Among others, a system which uses the wavelength multiplexity has a feature of being capable of transmitting a plurality of signals independently through one transmission line.
In the optical communication system, it is usual to convert a light signal to an electrical signal to reproduce or relay it in order to increase a transmission length or compensate for the attenuation of the light signal at a branch point. However, this method is not appropriate to the system which uses the wavelength multiplex because the respective waves must be separated for each reproduction or relay and they must be converted to electrical signals.
On the other hand, an optical amplifier which has recently been studied vigorously is expected to be appropriate to the wavelength multiplex system. The optical amplifier amplifies input waveforms independently as light signals. FIG. 1 shows a light-input/output characteristic. As seen from FIG. 1, a light output (or optical output) changes substantially in proportion to a light input (or optical input) until the light input reaches a certain level (called a linear region), and when the light input is above the certain level, the light output is substantially constant (called a saturated region).
FIG. 2 shows a wavelength dependency of a gain of the optical amplifier. As seen from FIG. 2, the gain is substantially constant from a wavelength .lambda.min to a wavelength .lambda.max.
Thus, the optical amplifier is used while taking the above characteristic into consideration. The optical amplifier includes a semiconductor laser amplifier which has a semiconductor laser structure and is driven by an injection current below a threshold, and an optical fiber amplifier. A gain of the semiconductor laser amplifier is controlled by the injection current while a gain of the optical fiber amplifier is controlled by a pumping light input.
In the optical amplifier, the gain control is effected in order to stabilize the light output. FIG. 3 shows a block diagram of an optical amplifier used in a prior art optical communication system. It uses a semiconductor laser amplifier as an optical amplifier and two waves .lambda..sub.1 and .lambda..sub.2 are wavelength-multiplexed on a transmission line. A light signal is applied to a semiconductor laser amplifier 72 through an optical fiber 71-1 which is used as a transmission line, and it is amplified thereby and a portion of the amplified light signal is applied to a photo-sensing circuit 76 through an optical power divider 73 while the remaining portion is applied to an optical fiber 71-2 which serves as a transmission line. The photo-sensing circuit 76 has an integration function having a sufficiently longer time constant than a pulse width of the signal light to produce a voltage having an amplitude corresponding to an average light intensity for a period in the order of the time constant. A gain control circuit 77 controls an output current of a driver 78, that is, a drive biasing current in accordance with an output voltage of the photo-sensing circuit 76.
The control is effected such that the output voltage of the photo-sensing circuit 76 is equal to a predetermined level.
In this manner, the light output of the semiconductor laser amplifier 72 when the light signal is applied thereto is always kept constant.
However, the prior art wavelength-multiplexed optical communication system which uses the optical amplifier has the following problems. Since the gain control of the optical amplifier is effected such that the total sum of the light intensities (or optical power) of all wavelengths applied to the optical amplifier is kept constant at the output end of the optical amplifier, it operates on a presumption that all wavelengths multiplexed are always applied to the optical amplifier. The lights are modulated by the transmission signal. Accordingly, when the number of wavelengths applied to the optical amplifier, that is, the number of wavelengths on the transmission line changes, the intensities (or optical power) of the respective wavelengths at the output end of the optical amplifier vary. An example of systems in which such a problem takes place is a packet communication system in which light signals appear in burst on the transmission line.
This is explained with reference to FIGS. 4A and 4B, which show light intensities (or optical power) of respective wavelengths at the output end of the optical amplifier in the optical amplification apparatus used in the prior art optical communication system. In the illustrated system, two wavelengths (.lambda..sub.1, .lambda..sub.2) are multiplexed. In FIG. 4A, only the wavelength .lambda..sub.1 is applied to optical amplifier, and in FIG. 4B both wavelengths .lambda..sub.1 and .lambda..sub.2 are applied. As described above, since the gain is controlled such that the light intensities of all wavelengths at the output end of the optical amplifier is kept constant, the light output for .lambda..sub.1 in FIG. 4A is double of that in FIG. 4B. Namely, depending on whether the wavelength .lambda..sub.2 is applied to the optical amplifier or not, the output light intensity of the other wavelength .lambda..sub.1 varies. The light intensity variation renders a permissible level of the system to the light level smaller. As a result, it is necessary to set a large dynamic range of the photo-detector. As a result, the stabilization effect of the light level on the transmission line by the use of the optical amplifier in the system is reduced. In the present example, the number of wavelengths multiplexed is two, but the light intensity variation increases as the number of wavelengths multiplexed increases.